The present invention relates a device for imposing a retardance between orthogonally polarized components of an optical signal without the use of birefringent material, and in particular to a virtual waveplate that can be used in an optical channel interleaver.
Optical multiplexers are used in the wavelength division multiplexing (WDM) of a plurality of optical channels for transmission via a single carrier fiber. At the receiving end of the fiber, the same general techniques are used to demultiplex the optical channels back into individual channels once again. Adding more channels to an optical signal increases the amount of data that can be sent down an optical network without the laying down of any additional fiber. The demand for additional capacity on current fiber networks keeps increasing, along with the demand for lower cost equipment to minimize initial capital costs, and less complicated equipment to reduce installation and maintenance costs.
One solution to the aforementioned problem includes the use of interleaver technology, and in particular to the birefringent waveplate-based interleaver technology disclosed in U.S. Pat. No. 4,566,761 issued Jan. 28, 1986; and U.S. Pat. No. 4,685,773 issued Aug. 11, 1987 both to Carlsen et al. Birefringent materials are relatively costly and temperature sensitive, moreover, there is a limit to the size that a single crystal can be grown, which could eventually restrict the capacity of the system. Other interleaver technologies are based on the Michelson interferometer, and include a Gires Tournois etalon in one arm (EP 933,657, filed Jan. 8, 1999 in the name of Dingel et al) or in both arms (U.S. Pat. No. 6,169,626 issued Jan. 2, 2001 in the name of Chen et al). Michelson Gires Tournois (MGT) interleavers are complicated to manufacture due to the high degree of accuracy required to match both of the arms thereof and to tune one or more cavities, which are temperature sensitive.
An object of the present invention is to overcome the shortcomings of the prior art by providing a less costly virtual waveplate that uses passive elements, which can be used in an interleaver device. Another object of the present invention is to increase the temperature stability of the waveplate assembly and to enable reflection losses to be minimized.
Accordingly, the present invention relates to a device for introducing a relative delay between orthogonally polarized components of an input signal comprising:
an input port for launching the input signal;
beam splitting means for dividing the input signal into first and second orthogonally polarized sub-beams, and for directing the first sub-beam along a first path and the second sub-beam along a second path;
first reflecting means in the first path for redirecting the first sub-beam back towards the beam splitting means;
delay means in the first path for imposing a relative delay between the first and second sub-beams;
second reflecting means in the second path for redirecting the second sub-beam back towards the beam splitting means for recombination with the first sub-beam; and
an output port for outputting the recombined first and second sub-beams.
Another aspect of the present invention relates to an optical filter comprising:
an first port for launching an input optical signal, which comprises a first and a second set of channels;
a second port for outputting the first set of channels;
a third port for outputting the second set of channels;
a first virtual waveplate; and
polarized beam separating means for directing the first set of channels to the second port, and for directing the second set of channels to the third port. The virtual waveplate comprises:
first beam splitting means for dividing the input signal into first and second orthogonally polarized sub-beams, and for directing the first sub-beam along a first path and the second sub-beam along a second path;
first reflecting means in the first path for redirecting the first sub-beam back towards the first beam splitting means;
second reflecting means in the second path for redirecting the second sub-beam back towards the first beam splitting means for recombination with the first sub-beam forming a first recombined signal; and
first delay means in the first path for imposing a first relative delay between the first and second sub-beams, whereby when the first and second sub-beams recombine, the first set of channels is orthogonally polarized relative to the second set of channels.